There is a general need for materials with low dielectric constants (low-k) in the integrated circuit manufacturing industry. Using low-k materials as the inter-metal and/or inter-layer dielectric of conductive interconnects reduces the delay in signal propagation due to capacitive effects. The lower the dielectric constant of the dielectric, the lower the capacitance of the dielectric and the lower the RC delay of the IC.
Low k dielectrics are conventionally defined as those materials that have a dielectric constant lower than that of silicon dioxide, that is k<˜14. Typical methods of obtaining low-k materials include doping silicon dioxide with various hydrocarbons or fluorine. These doping methods, however, generally cannot produce materials with dielectric constants lower than about 2.6. With more and more advanced technology needs, present efforts are focused on developing low-k dielectric materials with k less than 2.5. These ultra low-k dielectrics can be obtained by incorporating air voids within a low-k dielectric, creating a porous dielectric material.
Methods of fabricating porous dielectrics typically involve forming a composite film (sometimes referred to herein as a “precursor film”) containing two components: a porogen (typically an organic material such as a polymer) and a structure former or dielectric material (e.g., a silicon containing material). Once the composite film is formed on the substrate, the porogen component is removed, leaving a structurally intact porous dielectric matrix. Techniques for removing porogens from the composite film typically include, for example, a thermal process in which the substrate is heated to a temperature sufficient for the breakdown and vaporization of the organic porogen.
One issue with producing low-k porous materials relates to the fact that generally the more porous the material is (that is, the more air voids within the dielectric matrix), the lower the dielectric constant. In general, a lower dielectric constant is desirable. However, incorporating air voids may also diminish the film's mechanical strength and integrity, including increasing the film's intrinsic residual stress.
What are needed therefore are improved methods for forming porous dielectric low-k films with reduced tensile stress and improved mechanical integrity.